The present invention pertains to gas chromatography and more particularly to an on-line system and method which allows the analytes eluting from a chromatography column to be analyzed by traditional liquid detectors. Still more particularly, this invention relates to coupling a capillary gas chromatograph to a liquid radioactivity monitor.
There are several known techniques to separate a sample of a mixture into its various components. Separation techniques based on selective retention make up the general field of chromatography. Generally, separation in chromatography is due to redistribution of the molecules of a mixture between a mobile phase and a stationary phase which is held in a column. Gas chromatography is the branch of chromatography which uses a gas as the mobile phase. In gas chromatography the sample must be in vapor form in order for the gaseous mobile phase to carry the sample to the column. The components of the sample are then separated according to the rate at which the molecules of each component exchange with the stationary phase. Those components with the least affinity towards the stationary phase will elute from the column first.
The effluent from the column is continually monitored in order to detect when a component elutes from the column. Several types of detection systems are currently being used for gas chromatography. One type of detection system which is particularly useful when the sample contains isotopes which emit ionizing radiation, is a radiogas detector.
Radiogas detectors typically involve a catalytic combustion of a radiolabeled sample to form .sup.14 CO.sub.2 gas. This gas is then brought into contact with a scintillator matrix which is capable of converting radioactive energy into light. A detector is then used to count the number of light pulses over a period of time. The number of counts per unit of time changes in response to the presence of radiolabeled compounds which have been separated out by the gas chromatograph. Thus, as the analysis begins, the number of counts per minute is low representing only background radiation. Then when the first compound which emits ionizing radiation elutes from the column, the number of counts per minute quickly increases indicating the presence of the radiation. After the compound has completely eluted from the column and traveled past the counter, the number of counts per minute decreases back to the background level. This process continues until every compound separated by the capillary gas chromatography column has eluted from the column.
Radiogas detection systems have some significant drawbacks, however. First, radiogas detectors suffer from recovery loss as the catalytic combustion process is not 100% efficient. Furthermore, the efficiency of the combustion process changes over time as components of the combustion tube are depleted and/or contaminated. Accordingly, it is not possible to determine quantitative system recoveries. Secondly, because radiogas detectors use counting tubes having relatively large volumes to count the pulses of light, they are not capable of simultaneously providing high chromatographic resolution with high sensitivity. Resolution and sensitivity are directly related to the size of the counting tube in the detector. The larger the tube, the greater the sensitivity, but the less the resolution due to the diffusion of the gaseous molecules. Similarly, when the size of the counting tube is reduced in order to achieve better resolution, the sensitivity is decreased due to less scintillating material being present.
Both of these drawbacks could be improved if a liquid radioactivity monitor rather than a radiogas monitor could be used. Liquid radioactivity monitors (LRAM) are based on the same principle as radiogas monitors, but operate in the liquid rather than gaseous phase. Thus, the concentration of scintillating material can be increased, because the solubility of the scintillating material is much greater in a liquid phase than in a gaseous phase. Furthermore, diffusion rates are much slower in liquid phases than in gas phases, so liquid radiogas monitors maintain peak resolution better than the radiogas detectors. Accordingly, it would be advantageous to combine the separation powers of gas chromatography with the sensitivity and peak resolution of a liquid radioactivity detector. Malcolm Bowman and Morton Beroza, in Analytical Chemistry, Vol.40. No.3, pg 535, Mar. 1968, have reported using a packed column GC coupled to a spectrophotofluorometer via an open flowing interface. Their interface coupled the effluent from a GC to a mixing vessel which had a stream of solvent flowing through it, and an open tube which allowed the carrier gas to escape. Their interface also permits the more volatile analytes to escape with the carrier gas, however, preventing their detection. Furthermore, the resolution achieved is poor (e.g. a 0.2 .mu.g sample of fluorene, at an oven temperature of 150.degree. C. resulted in a peak which took over 2 minutes to completely elute). Thus, Bowman and Beroza's interface is not adequate for many applications.
It would be beneficial to combine a chromatography instrument with "real-time" liquid radioactivity monitoring via a closed transfer interface, so that sensitivity and resolution can be maintained. Such a combination is a primary objective of the present invention.
It is another objective of the present invention to provide an improved method for analyzing a sample containing particles which emit ionizing radiation.
It is a further objective of the present invention to provide an interface facilitating the coupling of gas chromatographic instruments to any detector designed for liquid streams.
Still another objective of the present invention is to provide a system which allows analytes in a gaseous stream to be continually derivatized facilitating detection.